You can not pull the update down from the servers, they have to send it to you (or you flash it yourself using one of the leaks... )

The "Check for Updates" does have a point though. Lets say you are in the car when you get the update, or in the middle of sending emails for work. Or even just out running errands. Basically... you are in a situation where you don't really have time to mess with a 15 minute update process.

Then you can decline the update and then go to the "Check for Updates" when you are ready.